In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a mask having a predetermined pattern, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film.
A resist material in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of pattern miniaturization. Typically, these pattern miniaturization techniques involve shortening the wavelength (and increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are starting to be introduced in the mass production of semiconductor elements. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a shorter wavelength (and a higher energy level) than these excimer lasers, such as electron beam (EB), extreme ultraviolet radiation (EUV), and X-ray.
As the wavelength of exposure light sources is shortened, the resist materials for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources. A known type of resist material that satisfies these conditions is a chemically amplified resist composition.
For example, in the case where the developing solution is an alkali developing solution (namely, an alkali developing process), a chemically amplified resist composition containing a resin component (base resin) which exhibits increased solubility in an alkali developing solution under the action of acid, and an acid generator is typically used. If the resist film formed using the resist composition is selectively exposed during formation of a resist pattern, then within the exposed portions, acid is generated from the acid generator component, and the action of this acid causes an increase in the solubility of the resin component in an alkali developing solution, making the exposed portions soluble in the alkali developing solution. The unexposed portions remain as a pattern, resulting in the formation of a positive-type pattern.
The aforementioned base resin uses a resin in which the polarity increases under the action of acid, resulting in an increase in the solubility of the resin in an alkali developing solution, but a decrease in the solubility of the resin within organic solvents. Accordingly, if a process that uses a developing solution containing an organic solvent (an organic developing solution) is employed (hereinafter also referred to as a solvent developing process or negative-type developing process) instead of the alkali developing process, then within the exposed portions of the resist film, the solubility in the organic developing solution decreases relatively, meaning the unexposed portions are dissolved in the organic developing solution and removed, whereas the exposed portions remain as a pattern, resulting in the formation of a negative-type pattern. Patent Document 1 proposes a negative-type developing process.
Currently, resins and the like that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are now widely used as base resins for chemically amplified resist compositions that use ArF excimer laser lithography or the like, as they exhibit excellent transparency in the vicinity of 193 nm.
Here, the term “(meth)acrylic acid” is a generic term that includes either or both of acrylic acid having a hydrogen atom bonded to the α-position and methacrylic acid having a methyl group bonded to the α-position. The term “(meth)acrylate ester” is a generic term that includes either or both of the acrylate ester having a hydrogen atom bonded to the α-position and the methacrylate ester having a methyl group bonded to the α-position. The term “(meth)acrylate” is a generic term that includes either or both of the acrylate having a hydrogen atom bonded to the α-position and the methacrylate having a methyl group bonded to the α-position.
The base resin generally includes a plurality of structural units in order to improve the lithography properties and the like. For example, in the case of a resin that exhibits higher polarity under the action of acid, a base resin is typically used that includes not only a structural unit having an acid-degradable group that degrades under the action of the acid generated from the acid generator, but also a structural unit having a polar group such as a hydroxyl group, and a structural unit having a lactone structure (for example, see Patent Document 2). In particular, introduction into the base resin of a structural unit having a lactone structure is very effective in terms of improving the adhesion of the resist film to the substrate during pattern formation and improving the affinity between a developing solution containing water (particularly in the case of an alkali developing process) and the resist film, and makes it easier to obtain favorable lithography properties and a superior pattern shape, and is therefore commonly used.
As a technique for further improving the resolution, a lithography technique called liquid immersion lithography (hereinafter also referred to as “immersion exposure”) is known, in which exposure (immersion exposure) is conducted in a state where the region between the objective lens of the exposure apparatus and the sample is filled with a liquid (an immersion medium) that has a larger refractive index than the refractive index of air (for example, see Non-Patent Document 1).
With this type of immersion exposure, it is considered that higher resolutions equivalent to those obtained using a shorter wavelength light source or a higher NA lens can be achieved using the same exposure light source wavelength, with no reduction in the depth of focus. Furthermore, immersion exposure can be conducted using existing exposure apparatus. As a result, it is expected that immersion exposure will enable the formation of resist patterns of higher resolution and superior depth of focus at lower costs. Accordingly, in the production of semiconductor elements, which requires enormous capital investment, immersion exposure is attracting considerable attention as a method that offers significant potential to the semiconductor industry, both in terms of cost and in terms of lithography properties such as resolution.
Immersion lithography is effective in forming patterns having various shapes. Further, immersion exposure is capable of being used in combination with currently studied super-resolution techniques such as phase shift methods and modified illumination methods. Currently, techniques using an ArF excimer laser as the exposure source are the most actively studied immersion exposure techniques. Further, water is mainly being investigated as the immersion medium.
Recently, a new lithography technique called a double patterning process has been proposed, in which a resist pattern is formed by conducting patterning two or more times (for example, see Non-Patent Documents 2 and 3). There are a number of varieties of double patterning processes, and examples include (1) a method in which a lithography step (from application of a resist composition through to exposure and developing) and an etching step are performed two or more times to form a pattern, and (2) a method in which the lithography step is repeated twice or more in succession. By using these double patterning processes, a resist pattern can be formed that has a higher resolution than a resist pattern formed using only a single lithography step (single patterning), even when a light source having the same exposure wavelength is used, and even when the same resist composition is used. Further, a double patterning process can be performed using an existing exposure apparatus.